Trying to go through tweets checking if one already exists twython

    for tweets in ExistingTweets:
ExistString = tweets['text']
ExistString = ExistString[:15]
if randomQuote[:16] == ExistString:
    randomQuote = AllQuotes[randint(0,5)].getText()
    randomQuote = randomQuote[printUntil:]

I am trying to see if the quote I am about to tweet has already been tweeted. The ExistString and randomQuote match up once(I am aware I'm calling :16 in the randomQuote, but somehow :15 and :16 equal to the same output) , but randomQuote doesn't change!

Any help is appreciated, thank you!